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There are three basic types of radiographic receptors. In addition to the
screen/film type described in this chapter there are the digital radiographic receptors described in
the section titled, "Digital Imaging Systems and Image
Processing," and the fluoroscopic systems that can also produce radiographs as described in
the section titled, "Fluoroscopic Imaging Systems."
In screen/film radiography, the receptor consists of the film mounted in
contact with either one or two intensifying screens, as shown below.
Intensifying screens are thin sheets, or layers, of fluorescent materials.
The screen-film combination is housed in either a cassette or a film
changer. The x-ray energy is absorbed by the intensifying screen material,
and a portion of it is converted into light. The light, in turn, exposes
the film. Intensifying screens are used because film is much more
sensitive to light than to x-radiation; approximately 100 times as much
x-radiation would be required to expose a film without using intensifying
screens. Unfortunately, intensifying screens introduce blurring into the
imaging process and places a limit on the visibility of detail that must
be considered when selecting screens for specific clinical applications.
A Conventional Screen/Film Radiographic Receptor
Different types of intensifying screens are available for clinical use. The selection of a screen for a specific procedure is usually based on a compromise between the requirements for image detail and patient exposure.
The receptor used for most radiographic procedures contains two intensifying screens mounted on each side of double-emulsion film. Using two screens in this manner increases x-ray absorption and receptor sensitivity
with the least amount of image blurring. In some procedures that require high image detail, such as mammography, one intensifying screen is used in conjunction with a single-emulsion film.
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The first function performed by the intensifying screen is to absorb the x-ray beam (energy) emerging from the patient's body. The ideal intensifying screen would absorb all x-ray energy that enters it; real intensifying screens are generally not thick enough to absorb all of the photons. As we discuss later, increasing the thickness of an intensifying screen to increase its absorption capabilities degrades image quality.
In most cases, a significant portion of x-ray energy is not absorbed by the screen material and penetrates the receptor. This is wasted radiation since it does not contribute to image formation and film exposure. The absorption efficiency is the percentage of incident radiation absorbed by the screen material. An ideal screen would have a 100% absorption efficiency; actual screens generally have absorption efficiencies
less than 100%. Absorption efficiency is primarily determined by three factors: (1) screen material, (2) screen thickness, and (3) the photon energy spectrum.
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The second function performed by the intensifying screen is to convert a portion of the absorbed x-ray energy into light. This is the fluorescent process. Fluorescence is the property of a material that enables it to absorb radiation energy in one portion of the photon-energy spectrum and emit some of the energy in the form of lower energy photons. Materials that glow, or emit visible light, when exposed to high-photon energy ultraviolet light have this property.
The figure below illustrates what happens to the x-ray energy that is absorbed by an intensifying screen. In the intensifying screen, the fluorescent process creates visible light when such material is exposed to high-energy x-ray photons. The intensifying screen is an energy converter; it converts approximately 5 to 20% of the absorbed x-ray energy into light. This percentage is the conversion efficiency of the screen, and depends on the type of material used in the screen.
Conversion of X-Ray Energy in an Intensifying Screen
Although the total energy of the light emitted by a screen is much less than the total x-ray energy the screen receives, the light energy is much more efficient in exposing film because it is "repackaged" into a much larger number of photons. If we assume a 5% energy conversion efficiency, then one 50-keV x-ray photon can produce 1,000 blue-green light photons with an energy of 2.5 eV each.
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Since film is more sensitive to light than to x-ray exposure, film can be exposed with much less radiation if an intensifying screen is used. Conventional x-ray film has an x-ray exposure sensitivity in the range of 50 mR to 150 mR
if exposed directly by the x-radiation.. When the film is combined with intensifying screens, the sensitivity ranges from approximately 0.1 mR to 10 mR, depending on the type of screen and film used.
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The sensitivity of a receptor, such as an intensifying screen-film combination, is expressed in terms of the exposure required to produce a film density of 1 unit above the base plus fog level. Some manufacturers do not provide sensitivity values for their receptor systems, but most provide speed values such as 100, 200, 400, etc. The speed scale compares the relative exposure requirements of different receptor systems. Most speed numbers are referenced to a so-called par speed system that is assigned a speed value of 100. Whereas sensitivity is a precise receptor characteristic that expresses the amount of exposure the receptor requires, speed is a less precise value used to compare film-screen combinations. There is, however, a general relationship between exposure requirements (sensitivity) and receptor speed values:
Sensitivity (mR) = 128/speed.
For example, a receptor with a true speed value of 100 requires an exposure of 1.28 mR to produce a 1-unit film density. Sensitivity and speed values are inversely related. A more sensitive receptor has a higher speed value than a less sensitive receptor. The range of receptor sensitivity and speed values used in radiography is shown below.
| Speed |
Sensitivity (mR) |
| 1200 |
0.1 |
| 800 |
0.16 |
| 400 |
0.32 |
| 200 |
0.64 |
| 100 |
1.28
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| 50 |
2.56 |
| 25 |
5.0 |
| 12 |
10.0 |
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Most receptors are given a nominal speed rating by the manufacturer. The actual speed varies, especially with
the x-ray spectrum (KV) and film processing conditions.
The sensitivity (speed) of an intensifying screen-film receptor depends on the type of screen and film used in addition to the conditions under which they are used and the film is processed.
We now consider characteristics of the screen that contribute to its sensitivity.
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Several compounds are used to make intensifying screens. The two major characteristics the material must have are (1) high x-ray absorption and (2) fluorescence. Because of their fluorescence, intensifying screen materials are often referred to as phosphors.
Soon after the discovery of x-rays, calcium tungstate became the principal material in intensifying screens and continued to be until the 1970s. At that time, a variety of new phosphor materials were developed; many contain one of the rare earth chemical elements. Phosphor compounds now used as intensifying screen materials include:
• barium lead sulfate
• barium strontium sulfate
• barium fluorochloride
• yttrium oxysulfide
• lanthanum oxybromide
• lanthanum oxysulfide
• gadolinium oxysulfide.
Each compound contains one element that is the primary x-ray absorber.
You will recall that the probability of x-ray absorption is higher when the photon energy is just slightly higher than the K energy of the absorbing material. The K-edge energy is, in turn, determined by the atomic number of the material.
Calcium tungstate, the most common screen material for many years, uses tungsten as the absorbing element. The K edge of tungsten is at 69.4 keV. For most x-ray examinations, a major portion of the x-ray beam spectrum falls below this energy. For this reason, screens containing tungsten are limited with respect to x-ray absorption. Today, most intensifying screens contain either barium, lanthanum, gadolinium, or yttrium as the absorbing element. The K edge of these elements is below a major portion of the typical x-ray beam spectrum. This increases the chance of x-ray interaction and absorption.
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The other elements in the compound contribute to the fluorescent properties of the material. Each compound produces light of a color (wavelength) that is specific to the particular material. The light from intensifying screens is produced in either the
ultraviolet, blue or green portion of the light spectrum, and intensifying
screens are sometimes classified as either blue or green emitters. The significance of this is that a screen must be used with a film that has adequate sensitivity to the color of light the screen emits. Some radiographic films are sensitive only to blue light; others (orthochromatic) are also sensitive to green light. If screen and film spectral characteristics are not properly matched, receptor sensitivity is severely reduced.
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The selection of a screen is generally a compromise between exposure and image quality, as illustrated
below. Thin screens absorb a relatively small fraction of the x-ray photons; thicker screens absorb a greater fraction and thus require less x-radiation to produce the same film exposure. Unfortunately, increasing screen thickness also increases image blur.
Effects of Screen Thickness on Image Blur
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The sensitivity of intensifying screens varies with x-ray photon energy because sensitivity is directly related to absorption efficiency. Absorption efficiency and screen sensitivity are maximum when the x-ray photon energy is just above the K edge of the absorbing material. Each intensifying screen material generally has a different sensitivity-photon energy relationship because the K edge is at different energies.
The spectrum of photon energies within an x-ray beam is most directly affected and controlled by the
KV; therefore, the sensitivity and speed of a specific intensifying screen is not constant but changes with
the KV selected for a specific procedure.
Significant exposure errors can occur if technical factors (KV and MAS) are not adjusted to compensate for the variation in screen sensitivity. This often occurs when the same technique charts are used with screens composed of different materials. Also, the
KV response characteristics of automatic exposure control (AEC) sensors should be matched to those of the intensifying screens.
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The most significant effect of intensifying screens on image quality is
that they produce blur. The reason for this was illustrated above. Let us consider the imaging of a very small object, such as a calcification. The x-ray photons passing through the object are absorbed and produce light along the vertical path extending through the intensifying screen. Before exiting the screen, the light spreads out of the absorption path, as illustrated. The light image of the object that appears on the surface of the intensifying screen is therefore blurred; the degree of blurring by this process is related to the thickness and
light transparency of the intensifying screen.
The major issue in selecting intensifying screens for a particular clinical application is arriving at an appropriate compromise between patient exposure and image quality or, more specifically, between receptor sensitivity (speed) and image blurring (visibility of detail). Screens that produce maximum visibility of detail generally have a low absorption efficiency (sensitivity) and require a relatively high exposure. On the other hand, screens with a high sensitivity (speed) cannot produce images with high visibility of detail because of the increased blurring.
Intensifying screens are usually identified by brand names, which do not always indicate specific characteristics. Most screens, however, are of five generic types:
1. mammographic
2. detail
3. par speed
4. medium speed
5. high speed.
The figure below shows how these general screen types fit into the relationship between image blur and required exposure.
General Relationship between Image Blur and Sensitivity (Speed)
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If the film and intensifying screen surfaces do not make good contact, the light will spread, as shown
below, and will produce image blurring. This is an abnormal condition that occurs when a cassette or film changer is defective and does not apply sufficient pressure over the entire film area. Inadequate film-screen contact usually produces blurring in only a portion of the image area.
Sources of Blur in Screen-Film Receptors
The conventional test for film-screen contact is to radiograph a wire mesh. Areas within the image where contact is inadequate will appear to have a different density than the other areas. This variation in image density is most readily seen when the film is viewed from a distance of approximately 10 ft and at an angle.
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If the film emulsion does not completely absorb the light from the intensifying screen, the unabsorbed light
from one side can pass through the film base and expose the emulsion on the other side. This is commonly referred to as crossover. As the light passes through the film base, it can spread and introduce image blur, as illustrated
above. Many modern film-screen receptor systems are designed to minimize crossover blurring. Crossover can be decreased by placing a light-absorbing layer between the film emulsion and film base, using a base material that selectively absorbs the light wavelengths emitted by the intensifying screens, and designing the film emulsion to increase light absorption.
It is not a significant problem with modern radiographic systems.
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When light encounters a boundary between materials, reflection can occur at the boundary surface. Reflections at boundaries between film emulsion, film base, intensifying screens, and cassette surfaces are known as halation and contribute to image blur. Single-emulsion films generally have a light-absorbing layer coated on the other side of the base to prevent halation.
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The amount of noise in radiographic images is affected, to some extent, by the characteristics of the intensifying screen; the crystal structure of the screen material produces a relatively small amount of image noise. Quantum noise is generally the most significant type of noise in radiographs. Intensifying screens with high conversion efficiencies generally produce more quantum noise than other screens for reasons discussed in
another chapter. Also, the visibility of noise is decreased, to some extent, by the blurring created within screens.
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Intensifying screens can be significant sources of image artifacts. Artifacts can be produced by scratches, stains, and foreign objects, such as hair, dust, and cigarette ashes, on the screen surface.
Intensifying screens should be cleaned periodically according to the manufacturer's instructions.
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